Selective hydrogenation of nitroaromatics to the corresponding n-arylhydroxylamine

ABSTRACT

Nitroaromatics are selectively hydrogenated in neutral media in the presence of precious metal catalysts and in the presence of dimethylsulfoxide to produce N-arylhydroxylamines in high yield.

United States Patent Rylander et al. 1 Sept. 26, 1972 [54] SELECTIVEHYDROGENATION OF NITROAROMATICS TO THE References C d CORRESPONDING N-UNITED STATES PATENTS ARYLHYDROXYLAMINE 2 894 036 7/1959 0 h 260/58 0[721 lnvemrs= Paul Newark; Irene 3,491,151 1/1970 8236?? ..260/584Karpenko, lrvmgton; George R. Pond Newark of Primary Examiner-JosephRebold 73 A I E lh d h Assistant Examiner-Donald M. Papuga sslgnee zgszz men s C emlca s Attorney-Miriam W. Leff and Samuel Kahn [22] Filed:April 16, 1970 [57] ABSTRACT [21] Appl. No.: 29,055 Nitroaromatics areselectively hydrogenated in neutral media in the presence of preciousmetal catalysts and in the presence of dimethylsulfoxide to produce N-[52] US. Cl ..260/57 8, 260/580 Ih d I h 1d 511 Int. Cl ..C07c 87/48 myy my ammes m [58] Field of Search ..260/578, 580, 5 83 DD 2 Claims, 4Drawing Figures SELECTIVE HYDROGENATION OF NITROAROMA'IICS TO THECORRESPONDING N- ARYLI-IYDROXYLAMINE BACKGROUND OF THE INVENTION Thisinvention relates to an improved method for selectively reducingnitroaromatics to produce N-arylhydroxylamines in high yield. TheN-arylhydroxylamines are compounds in which the group -NI-IOI-I isattached to an aromatic nucleus. The invention is particularlyapplicable to the selective reductionof nitrobenzenes, and forconvenience, the discussion below is mainly directed to the selectivereduction of nitrobenzenes to produce N phenylhydroxylamines.

It is well known that nitrobenzenes are reduced over platinum metalcatalysts, usually in highyields, to

anilines. N-phenylhydroxylamines, which may be formed as intermediatesinthe reduction do not appear as major constituents in the reaction mediabecause of the competing reactions leading to anilines. These completingreactions involve the further reduction of the intermediates or thedirect reduction of the nitrobenzenes without the formation of theintermediate phenylhydroxylamines. When there is less than a substantialformation of the desired intermediate product, production of thesecompounds from nitrobenzenes isimpractical because of the difficultiesof separation and purification of the products.

It is thus an object of the present invention to provide an improved andpractical process for the production of N-phenylhydroxylamines fromnitrobenzenes by catalytic hydrogenation. It is a particular object ofthis invention to provide a process for the selective catalytichydrogenation of nitrobenzenes in which N-phenylhydroxylamines areformed in the reaction media in substantial amounts. It is a furtherobject to provide a catalytic process for selectively hydrogenatingnibrobenzenes in which there is a significant drop in the hydrogenationrate when two molar equivalents of hydrogen are absorbed, thusfacilitating the production of N-phenylhydroxylamines in good yields.

These and other objects are accomplished by the hydrogenation ofnitrobenzenes in neutral media in the presence of a precious metalcatalyst and a selectivity agent.

THE INVENTION In accordance with this invention an improved method isprovided for producing N-arylhydroxyamines from nitroaromatic compoundscomprising selectively hydrogenating nitroaromatic compounds in neutralmedia in the presence of a platinum group metal catalyst and in thepresence of dimethylsulfoxide (DMSO).

The selectivity agent is the DMSO. It has been found that even verysmall additions of DMSO to the reaction mixture have a potent effect onthe selectivity of the hydrogenation. While the amount of DMSO that canbe employed can be varied considerably, there is a limiting value beyondwhich added amounts of DMSO have relatively little effect. Preferablythe DMSO is present in an amount ofv about 0.1 mole to 100 moles permole of catalyst metal.

The effect of DMSO as an additive in the hydrogenation of nitrobenzenecan readily be seen byreference to the accompanying figures, which aregraphical presentations of the results of experiments described below.

FIG. 1 is a graph of product composition plotted against the amount ofDMSO added. It shows the effect of the amount of DMSO on the yield ofphenylhydroxylamine with other variables held constant.

FIG. 2 gives two curves in which the H absorption is plotted againsttime. The curves compare the rate of H absorption of a prior art system(using no DMSO) with that of the present invention, i.e. employing DMSO,and show 2 effect of the addition of DMSO On the hydrogenation rate.

FIG. 3a shows the product composition as a function of the H absorbed ina prior art system FIG. 3b is comparable to 3a except that DMSO ispresent in the system. A comparison of FIGS. 3a and 3b shows the effectof the addition of DMSO on the selectivity of the reaction.

The preferred catalyst for the present invention is platinum both withrespect to the yield of arylhydroxylamines and the rate of hydrogenabsorption. The platinum may be used unsupported, e.g. as the oxide,supported on a suitable carrier material, e.g. carbon, and/or incombination with other metals, e.g. an alloy or mixture of platinum andgold. Other platinum group metals may be used, including palladium,rhodium, iridium, and ruthenium, their oxides, and combinations thereof.Suitable supports include carbon, alumina, calcium carbonate, bariumsulfate, preferably carbon. The catalyst concentration on the supportmay be from about 0.05 percent to 20 percent by weight, based on theweight of support; preferably from about 1 percent to 10 percent. Thecatalysts can be used repeatedly. Any of the catalyst preparations maybe used. One method, for example, for preparing a supported platinumcatalyst involves treating the carrier particles, which may be powders,spheres, granules, extrudates, etc., with an aqueous solution of awater-soluble compound of platinum and reducing the compound, depositingit thereby on the carrier, by contact with a reducing agent, e.g. ahydrogen stream. Generally the catalyst metal is present in the reactionmixture in an amount of from about 0.01 to 20 percent, based on theweight of the substrate, preferably 0.5 to 5 percent. The amount of thecatalyst affects the rate of hydrogenation. Generally when less thanabout 0.01 percent of the catalyst metal is used, the rate is too low tobe practical; and at concentrations above about 5 percent the smallincrease in rate does not justify the increased cost.

The selective reduction is carried out in neutral media. Suitably it iscarried out in a liquid phase solution of the nitrobenzene substratedissolved in a suitable inert neutral solvent. Examples of suitablesolvents are: lower aliphatic'alcohols such as methanol, ethanol,isopropanol, propanol n-butanol, t-butanol; liquid hydrocarbons such asbenzene, hexane, heptane, cyclohexane, toluene, octane, xylene. Watermay be used. Preferred solvents are lower aliphatic alcohols. Withoutsolvent the reduction does not go as smoothly. It is more difficult tocontrol the heat dissipated and coupled products tend to form. Too muchsolvent complicates recovery. Preferred ranges are from about 25 percentsolvent to 99 percent solvent based on the weight of the substrate.

The hydrogenation may be conveniently carried out at ambient temperatureand one atmosphere of hydrogen. However, the temperature may range fromabut about to 150C, preferably about 25 to C. pressures from about 0.1atm to 100 atm may be used. It is, of course, more convenient to operateat or above 1 atmosphere and no advantage is seen in using pressureshigher than 100 psig, thus permitting the use of low pressure equipment.

Generally reaction rates increase with increased temperature andpressure. However, selectivity tends to decrease with increasingtemperature, and higher costs become a factor with more severeconditions. It will be appreciated, therefore, that the conditionsclosest to ambient which achieve the desired result are usedadvantageously.

Operating under the above conditions including the presence of DMSO, thereaction proceeds until two molar equivalents of hydrogen are absorbed.At this point the rate of hydrogen absorption slows down markedly.Contrastingly, without DMSO the rate is constant over the absorption of3 moles of hydrogen. The marked decrease in the rate of hydrogenabsorption after 2 moles of hydrogen are absorbed facilitates stoppingthe reaction at the proper time for maximum arylhydroxylamine recovery.

The process is operated batchwise or continuously using conventionalequipment. On absorption of about 2 molar equivalents of hydrogen or ata time when the hydrogen absorption rate decreases, or after suitablecontact time (in a continuous process), the hydrogenation isinterrupted, and the product N-arylhydroxylamine is recovered. Forexample, on absorption of 2 molar equivalents of hydrogen the catalystis removed by filtration and the product recovered by known methods suchas dilution with water, saturation with salt, cooling to 0C., andremoving the precipitated arylhydroxylamine by filtration.

The nitroaromatic compounds which may be employed as substrates in theprocess of this invention will be obvious to those skilled in the art.Typically they are nitrobenzene, nitronaphthalene, and derivativesthereof. With respect to the derivative compounds, generally, theprocess is applicable to mononitro and dinitro aromatics, but monoderivatives are preferred. Dinitro compounds, as 2, 4-dinitrotoluene,give a mixture of products. Aromatic nitro compounds having other easilyreduced groups, as 4-nitrostyrene, undergo secondary changes and produce4-ethyl-N-pheny1- hydroxylamine. Compounds, like 3- nitroacetophenone,with functions reactive to hydroxylamines may yield condensationproducts. Examples of derivative compounds suitable as substrates forour invention are alkyl substituted aromatic nitro compounds, as4-nitrotoluene, 3-nitroethylbenzene, 2- methyl-4-nitrotoluene;halonitroaromatics as 3- chloronitrobenzene, 4-fiuoronitrobenzene, and5- chloro-l-nitronaphthalene; aminonitroaromatics, as 4-aminonitrobenzene, 3-N-methylaminonitrobenzene, and 4-N,N-dimethy1aminonitrobenzene; aromatic nitronitriles, as3-nitrobenzonitri1e, and 4-nitrobenzylcyanide; aromatic alcohols, as3-nitro-l-phenylethanol, 4-nitro-2-phenylethanol, and 4-nitrobenzylalchohol; aromatic nitrophenols as, 4-nitrophenol and 3-nitro-5-methylphenol; aromatic nitro ethers, as 2-nitroanisole and3-nitrophenetole. Arylhydroxylamines such as N phenylhydroxylamines arevaluable compounds for producing arninophenols. It is well known, forexample, that the phenylhydroxylamines rearrange in acid, e.g., dilutesulfuric acid, to the aminophenols, which are important commercially inthe production of dyes, antioxidants, developers, pharmaceuticals, andmany other products. henylhydroxylamine is an intermediate in thepreparation of cupferron, an analytical reagent.

The following examples are given to illustrate the present invention. Inthe experiments the same general procedure was employed, as follows:

A glass vessel was changed with substrate, solv'ent, catalyst, andvarious amounts of DMSO. The vessel was placed in a shaker and thereaction mixture was shaken while at ambient temperature under 1atmosphere of hydrogen. The reaction was interrupted at various pointsin the reduction, the mixture filtered and the filtrate analyzed by gaschromatography.

EXAMPLE 1 In a series of experiments 2 ml of nitrobenzene, 100 ml ofpercent ethyl alcohol and various amounts of catalysts were charged to areactor. To this reaction mixture varying amounts of dimethylsulfoxideranging from 0 to 2 ml were added. After 2 molar equivalents of hydrogenwere absorbed the reaction was stopped and the filtrate was analyzed.Typical results are given in TABLE I and FIG. 1. FIG. 1 is a plot of theproduct composition against the amount of DMSO added in the tests using5 percent Pt on carbon as the catalyst. The results in Table I show thatPt is a preferred catalyst for the selective hydrogenation ofnitrobenzene. The marked effect of even a small amount of added DMSO onthe product composition is shown dramatically in Table l and FIG. 1.

TABLE I Hydrogenation of Nitrobenzene 100 ml 80% aqueous ethanol, 2 mlnitrobenzene, ambient temperature, one atmosphere hydrogen) ProductAnalysis, Mole Amount of Weight dimethyl of sulfoxide hydroxyl- PhenylNitro- Catalyst Catalyst added amine Aniline benzene 5% Pd/C 30 mg none4.8 68.2 27.0 5% Rh/C 100 mg none 18.0 57.9 24.1 5% lr/C 250 mg none 9.963.2 26.9 5% Ru/C C 500 mg none 1.4 65.3 33.3 5% Os/C 1000 mg none 2.762.7 34.6 5% Pt/C 100 mg none 24.7 53.1 22.2 5% Pt/C 100 mg 0.125 ml67.6 26.0 6.4 5% Pt/C 100 mg 0.25 ml 76.9 20.4 2.7 5% Pt/C 100 mg 0.50ml 80.4 16.8 2.8 5% Pt/C 100 ml 1.00 ml 86.0 13.1 0.9 5% Pt/C 100 mg1.50 ml 85.7 12.8 1.5 5% Pt/C 100 mg 2.00 ml 84.6 13.9 1.5 1% Pt/C 250mgnone 47.2 41.1 11.7 1% Pt/C 250 mg 0.5 m1 77.6 18.6 3.8 1% Pt/C 250 mg1.0 ml 84.3 13.0 2.7 1% Pt/C 250 mg 1.5 ml 85.2 13.0 1.8 1% Pt/C 250 mg2.0 ml 848 13.5 1.7 PtO 15 mg none 2.5 68.7 28.8 HO,- A11 0, :10 15 mgnone 3.2 69.2 27.6 Flo,- 2 n 90:10 15 mg 1.0 mi 53.5 44.2 2.3

EXAMPLE 2 In a series of experiments 2 ml of nitrobenzene, 100 ml. of 80percent ethyl alcohol, 100 mg. of 5 percent Pt on carbon, and variousamounts of DMSO were charged to a reactor. In these tests the rate ofhydrogen absorption and the product composition on absorption of variousincrements of hydrogen were determined. The product composition wasdetermined by gas chromatography. Typical results are tabulated in TableI and FIGS. 2, 3a, and 3b.

In Table II: The tests of the group labeled A show the effect of theconcentration of DMSO on the rate of hydrogenation and the productcomposition. The tests of group B show the rate of hydrogenation and theproduct composition after absorption of successive increments ofhydrogen in a hydrogenation system according to the prior art, i.e.,with no DMSO present. Group C is comparative to group B, except that 1ml of DMSO is present, in accordance with the present invention.

FIG. 2 is a graph of hydrogenation rates obtained in comparative testsin which no DMSO and 1 ml of DMSO were used. Curve B represents theresults with the prior art system, i.e., without DMSO. Curve Crepresents the results with 1 ml of DMSO added.

FIG. 3a is increased temperature of pressure. composition as a functionof hydrogen absorbed temperature, a prior art system, using no DMSO.FIG. 3b is a plot comparable to that of 3a except that 1 ml of DMSO ispresent in the reaction mixture.

The results in TABLE 11 show that increasing quantities of DMSO have astriking effect on the rate of hydrogenation. Initial small additions ofDMSO sharply increase the rate whereas further larger additions haverelatively little effect. Group C tests (with 1 ml DMSO) of TABLE II andFIG. 2 show that the rate of hydrogenation is sharply limited afterabsorption of 2 moles of hydrogen, i.e., the rate of hydrogenation ofphenylhydroxylamine is decreased relatively more than for nitrobenzene.With 1 ml DMSO, the rates of hydrogenation for the first 2 moles and fora third mole are 19.2 ml H /min and 2.7 ml H /min, respectively. WithoutDMSO the rate is constant over 3 moles. As indicated above, the sharpdecline in rate after 2 moles of hydrogen are absorbed facilitates therecovery of a maximum amount of the intermediate product.

The marked effect of DMSO on the selectivity of the hydrogenation isclearly shown in the comparison of FIGS. 3a and 3b. Without DMSO, themaximum yield of phenylhydroxylarnine is about 23 percent; (FIG. 3a)whereas in its presence the maximum yield is percent (FIG. 3b).

Variations and modifications which will be obvious and apparent to thoseskilled in the art may be made in the invention without departing formthe spirit and scope thereof.

TABLE II ml 80% ethanol, 2 ml mtrobenzene, ambient temperature, oneatmosphere hydrogen) Product Analysis DMSO H, Absorbed Nitro- TestsAdded Moles time PHA Aniline benzene None 2.00 23'50" 24.7 53.1 22.20.125 ml 2.00 37'35" 67.6 26.0 6.4 0.25 2.00 44'25" 76.9 20.4 2.7 A 0.502.00 47'15" 80.4 16.8 2.8 1.00 2.00 61'31" 86.0 13.1 0.9 1.50 2.0061'07" 85.7 12.8 1.5 2.00 2.00 74'20" 84.6 13.9 1.5 None 0.25 3'23" 7.52.2 90.3 None 1.00 13'36" 18.6 22.6 58.8 None 1.50 1530' 20.6 37.4 42.0B None 19'03" 24.5 42.6 32.9 None 2.00 23'50" 24.7 53.1 22.2 None 2.2525'09" 21.7 67.9 10.4 None 2.50 3000" 15.0 81.9 3.1 1.00 0.50 12' 57"30.1 0 69.9 1.00 1.00 24'30" 50.0 0.9 49.1 1.00 1.50 34'09" 65.9 5.228.9 C 1.00 1.75 45'00" 75.4 9.1 15.5 1.00 2.00 61'31" 86.0 13.1 0.91.00 2.25 81'11" 71.6 28.4 0 1.00 2.50 119'08" 50.0 50.0 0

What is claimed 1s: 1. In a process for the reduction of an aromaticnitrocompound to produce the corresponding N-arylhydroxylamine wherein asolution of the aromatic nitrocompound in a neutral solvent is contactedwith hydrogen at a temperature of from about 0 C. to about C. and apressure of from 0 to 100 psig in the presence of a platinum-containingcatalyst until about 2 mols of hydrogen per mol of aromaticnitrocompound are absorbed and the N-arylhydroxylamine is separated fromthe reaction mixture, the improvement which comprises effecting saidcontacting in the added presence of from 0.1 to 100 mols per mol ofcatalyst metal, of dimethylsulfoxide.

2. The process as defined in claim 1 wherein the aromatic nitrocompoundwhich is reduced is nitrobenzene and phenylhydroxylamine is recovered.

2. The process as defined in claim 1 wherein the aromatic nitrocompoundwhich is reduced is nitrobenzene and phenylhydroxylamine is recovered.